The invention relates to a method for producing a bore in work pieces which have already been prebored, especially for producing the hub bore in prebored railroad wheels.
The production or rebuilding of wheel sets, such as railroad wheel sets, requires uniting both of the wheel disks with the axle to assemble the wheel set. To accomplish this, it is necessary that both hub seats of the axle or shaft and the hub bore in each wheel disk have very specific, correlated diameters for cooperating with one another in an operatively fitted manner. Thus, the dimensions of the mentioned diameters must be accomplished with a high degree of precision, in order to assure the functioning of the desired fitting.
When producing the wheel disks of a railroad wheel set, the entire outer surface of the wheel disks is machined by cutting, whereby the work piece is heated substantially. The last step of the machining is to bore out the hub bore to an operatively fitting dimension. For this purpose, the finish machining of the hub bore must occur at the end of the entire operation, so that damage to the outer surface of the bore, e.g., by chips, is avoided. The boring tool is moved to the desired operatively dimensioned diameter, for example, by means of a numerically controlled carriage or support. During such finish machining the work piece has the very high temperature originating from the rough machining. If the bore of the warm work piece is now bored out with this tool setting, then the bore would be too small after the work piece has cooled down to room temperature or to an operating temperature. If the wheel disk is assembled with the axle while the wheel disk is still in a heated condition, the wheel disk would be subject to such high internal stresses, due to the subsequent cooling, that it would crack after a short time of operation.
The only possible way to avoid these disadvantages according to the prior art, is to allow the work pieces to cool off to room temperature, and only then to do the finish machining. However, this requires costly floor space for an intermediate storing. Consequently, the work pieces are sometimes stored intermediately outside in the stock yard or in an unheated shed section. The machined axles, however, do not require intermediate storing because they do not heat up as much as the wheel disks. Hence, the wheel disks may be cooled off too much for machining immediately after the intermediate storing period, especially during colder seasons or in colder areas of the globe. The wheel disks have a temperature, which is well below the axle or shaft temperature, since the axles have been exclusively in a heated hall. A machining of the wheel hub bore is also not possible in this condition, because a correct operatively fitting dimension may not be attained under these conditions. The proper fit would loosen as a result of heating of the wheel disk or a cooling of the axle.